Marine trunk piston engines generally use Heavy Fuel Oil (‘HFO’) for offshore running. Heavy Fuel Oil is the heaviest fraction of petroleum distillate and comprises a complex mixture of molecules including up to 15% of asphaltenes, defined as the fraction of petroleum distillate that is insoluble in an excess of aliphatic hydrocarbon (e.g. heptane) but which is soluble in aromatic solvents (e.g. toluene). Asphaltenes can enter the engine lubricant as contaminants either via the cylinder or the fuel pumps and injectors, and asphaltene precipitation can then occur, manifested in ‘black paint’ or ‘black sludge’ in the engine. The presence of such carbonaceous deposits on a piston surface can act as an insulating layer which can result in the formation of cracks that then propagate through the piston. If a crack travels through the piston, hot combustion gases can enter the crankcase, possibly resulting in a crankcase explosion.
It is therefore highly desirable that trunk piston engine oils ('TPEO's) prevent or inhibit asphaltene precipitation. The prior art describes ways of doing this.
WO 96/26995 discloses the use of a hydrocarbyl-substituted phenol to reduce ‘black paint’ in a diesel engine.
Cashew nut shells contain approximately 40% phenolic compounds. They are readily available worldwide and potentially constitute a low-cost, widely-available and renewable raw material for phenols. “Stabilization of Asphaltenes by Phenolic Compounds Extracted from Cashew-Nut Shell Liquid” Moreira, Lucas and Gonzalez, Journal of Applied Polymer Science, Vol. 73, 29-34 (1999) (herein “Moreira et al”) describes the use of phenolic compounds extracted from cashew nut shell liquid (herein “CNSL”) for the stabilisation of asphaltenes, but in crude oil. Specifically, Moreira et al show that technical CNSL (obtained by roasting the shells to extract liquid) and cardanol (obtained by distilling technical CNSL) are both effective at stabilizing asphaltenes in toluene/heptane mixtures.
Moreira et al do not, however, address the problem of asphaltene precipitation in TPEO's, which have a different properties compared with crude oil. They remark that technical CNSL and cardanol contain phenolic compounds with long linear alkyl chains containing 15 carbon atoms with variable unsaturation degrees, meta-substituted in the aromatic ring. They further remark that side-chain unsaturation seems an attractive possibility to improve the efficiency of phenolic compounds to disperse asphaltic dispersions.
“Chemistry and Technology of Lubricants” (Second Edition) edited by Mortier and Orszulik discusses the properties and formulation of marine lubricants (paragraph 10.6) and summarises typical properties of the three types of marine diesel engine lubricants, i.e. system oil, cylinder oil and TPEO. It notes that the three types of oils have quite different performance requirements and summarises these also. Paragraph 10.9 discusses TPEO's in more detail such as issues arising from their use in engines operating on HFO.